A submersible well pump assembly include a rotary pump driven by an electrical motor. A seal section operably connects between the motor and the pump for reducing a pressure differential between motor oil in the motor and well fluid surrounding the pump assembly. The pump assembly contains a sealed fluid. A sensor mounted to the well pump assembly detects contamination of the sealed fluid by well fluid encroaching into contact with the sealed fluid. The sensor may be mounted in the motor or the seal section to detect well fluid contamination of motor oil. The sensor may be mounted in a secondary pump that has that has temporary barriers to block the entry of well fluid into a buffer fluid contained in the secondary pump while the secondary pump is in a storage condition within a well.